1. Field of the Invention
The present invention relates to a transmission power control method of a wireless communication terminal and a base station therefor. More particularly, the invention relates to a method of controlling transmission power of a terminal station in a wireless communication system having a plurality of base stations and a plurality of terminal stations such as portable telephones and a base station for generating a power control signal for the method.
2. Description of the Related Art
An example of a method of controlling transmission power of a terminal station in a CDMA cellular communication system is described in Section 6.1.2.3.2 (Chapter 6, p 6), Section 6.6.6.2.7.2 (Chapter 6, p 180), and Section 7.1.3.1.7 (Chapter 7, p 13 to p 15) of document of the standard of revision TIA/EIA/IS-95-A issued by TIA (Telecommunications Industry Association) on May 18, 1995. Similar methods are also described in Section 2.1.2.3.2 (Chapter 2, p 36 to p 38), Section 3.1.3.1.10 (Chapter 3, p 97 to p 99), and so on of document of the standard of revision STD-T64-C.S0002-A issued by ARIB (Association of Radio Industries and Business) on Mar. 2, 2000, and Section 9.2.1.2.4 (Chapter 9, p 23 to p 26), Section 9.2.1.4.2 (Chapter 9, p 53 to p 54), and so on of document of the standard of revision TIA/EIA/IS-856 issued by TIA on Oct. 27, 2000.
According to the power control methods, when a received power of a radio wave transmitted from a certain terminal station is lower than a power necessary to perform a proper communication, each base station transmits a power control signal for instructing the terminal station to increase the transmission power (hereinbelow, simply called “power-up control signal”). When the received power is higher than the power necessary to perform a proper communication, the base station sends a power control signal for instructing the terminal station to decrease the transmission power (hereinbelow, simply called “power-down control signal”).
A terminal station decreases its transmission power if there is even one power-down control signal in power control signals transmitted from a base station, and increases its transmission power when only the power-up control signals are received. By the method, the transmission power of a terminal station is controlled so that a base station which receives radio waves transmitted from the terminal station with the highest power receives the radio waves from the terminal station with a sufficient power to perform a proper communication. For example, in a CDMA cellular communication system, as shown in FIG. 1, communications between terminal stations 111 to 118 and base stations 101 to 103 are performed wirelessly so that the terminal stations 111 to 118 can freely move. Communications between the base stations 101 to 103 and a base station control device 100 can be performed in a wired or wireless manner. Further, the base station control device 100 is connected to another communication system via a switching network and/or the Internet 120.
Radio waves transmitted from a terminal station are received with a high power by a base station close to the terminal station and received with a low power by a remote base station. Therefore, the radio waves transmitted from a terminal station within a zone close to a base station are received with a power higher than that received by another base station. The zone is called a cell of the base station. A cell boundary exists at a point nearly equidistant from neighboring base stations. In FIG. 1, cell boundaries are expressed by alternate long and short dash lines. A cell boundary between the base stations 101 and 102 is indicated by 151, a cell boundary between the base stations 102 and 103 is indicated by 152, and a cell boundary between the base stations 103 and 101 is indicated by 153. The communication in the direction from a base station toward a terminal station will be called “downlink” communication, and the communication in the opposite direction will be called “uplink” communication.
In a conventional method of controlling the power of a terminal station, for example, when the power of the uplink radio wave transmitted from the terminal station 111 and received by the base station 101 in the cell of the base station 101 does not reach the power necessary to perform a proper communication, the base station 101 transmits the power-up control signal to the terminal station 111. Since the terminal station 111 is within the cell of the base station 101, a power received by other base stations is lower. Therefore, the other base stations send the power-up control signal to the terminal station 111. All the power control signals received by the terminal station 111 give instructions to increase the transmission power. Based on the power-up control signal, the terminal station 111 increases the transmission power.
If the power of uplink radio waves transmitted from the terminal station 111 and received by the base station 101 exceeds the power necessary to perform a proper communication, the base station 101 transmits the power-down control signal to the terminal station 111. Then, at least one of the power control signals received by the terminal station 111 gives an instruction to decrease the transmission power irrespective of the power control signals generated by the other base stations. Based on the power-down control signal, the terminal station 111 decreases the transmission power. That is, irrespective of the powers received from the other base stations, the transmission power of an arbitrary terminal station 111 in the cell of the base station 101 is controlled to a power necessary and sufficient to perform a communication in which the power received by the base station 101 is proper.
The terminal station 111 in the cell of the base station 101 can usually receive the downlink radio waves transmitted from the base station 101 with a higher received power than the downlink radio waves transmitted from the other base stations. Therefore, between the terminal station 111 and the base station 101, a downlink communication path by which a communication can be performed more efficiently as compared with a communication path between the terminal station 111 and another base station is established. Between the terminal station 111 and the base station 101, an uplink communication path capable of transmitting information at a sufficient received power is also formed. It is also avoided that the terminal station 111 transmits radio waves with the power higher than required.
In the above-described method of controlling the transmission power of a wireless communication terminal, when a terminal station is positioned in the vicinity of cell boundaries of a plurality of base stations, an inconvenient case may occur such that a base station which can perform a communication with the terminal station most efficiently in the uplink communication and that in the downlink communication are different from each other, for the following reason. For example, the following case may occur in which downlink radio waves received by the terminal station can be received with a higher power when they are sent from the first base station than the second base station. However, uplink radio waves can be received with a higher power when they are received by the second base station than the first base station. A main cause of such a case is that, due to different frequencies of radio waves, an influence of multi-wave interference of radio waves in the uplink communication and that in the downlink communication are different from each other.
To simply the explanation, the following case is assumed. As shown in FIG. 1, a terminal station 119 exists in the vicinity of the cell boundary 151 between the base stations 101 and 102, a path 261 of direct waves and a path 262 of reflected waves exist between the terminal station 119 and the base station 101, and only a path 263 of direct waves exists between the terminal station 119 and the base station 102. The radio waves of a frequency used in the uplink communication and that in the downlink communication are different from each other, and the difference in path lengths between the path 261 of direct waves and the path 262 of reflected waves is an integral multiple of the wavelength of the downlink radio wave and is an odd multiple of the half of the wavelength of the uplink radio wave. When the radio wave is reflected by a reflection obstacle 231, phase inversion does not occur. In such a case, although the downlink radio waves received by the terminal station 119 can be received with a power higher when they are transmitted from the base station 101 as compared with the case where the radio waves are transmitted from the base station 102, the uplink radio waves can be received by the second base station 102 with a higher power as compared with the case where the radio waves are received by the base station 101.
That is, with respect to the downlink radio waves from the base station 101 which are received by the terminal station 119, the phase of the direct wave and that of the reflected wave coincide with each other. Consequently, the received power is higher as compared with the case where there is no reflected wave. With respect to the uplink radio waves from the terminal station 119 which are received by the base station 101, the phase of the direct wave and that of the reflected wave are opposite to each other. Consequently, the received power is lower as compared with the case where there is no reflected wave.
The terminal station 119 is in the vicinity of the cell boundary 151. That is, the received power of the uplink radio waves from the terminal station 119 which are received by the base station 101 and that received by the base station 102 become equal to each other if there is no reflected wave. Due to the influence of the reflected wave, therefore, the uplink radio wave can be received by the base station 102 with a higher power as compared with the case where it is received by the base station 101. Similarly, the downlink radio wave can be received by the base station 101 with a higher power as compared with the case where it is received by the base station 102. That is, the base station by which the communication with the terminal station 119 can be performed most efficiently in the uplink communication and that in the downlink communication are different from each other.
In an actual wireless communication system, a number of reflection obstacles for the radio waves exist, and their shapes and reflectance are various. Therefore, in many cases, a number of paths of reflected waves exist, and the intensities of the waves are various. When an obstacle for the radio waves exists, the direct wave may be attenuated or vanished. Further, many of the reflection obstacles and obstacles for the radio waves change their shapes and positions momentarily. In other words, as a result of the multi-wave interference caused under various conditions, the following case occurs in various situations, in which the downlink radio waves transmitted from the base station 101 can be received with a higher power as compared with those transmitted from the base station 102, and the uplink radio waves can be received by the base station 102 with a higher power than the base station 101.
In this case, in the conventional technique, the transmission power of the terminal station 119 is controlled so that the power received by the base station 102 which receives the uplink radio waves from the terminal station 119 with the highest power becomes equal to the power necessary and sufficient to perform a proper communication. However, the power received by the base station 101 becomes insufficient to perform a proper communication. Therefore, when an uplink signal transmitted from the terminal station 119 is received by the base station 101, an error occurs with the high probability.
On the other hand, the downlink radio waves can be received with a higher power when they are transmitted from the base station 101 than the base station 102. Consequently, it is necessary to transmit the downlink radio waves from the base station 101 to perform a communication at the highest transfer rate. The terminal station 119 therefore requests data transmission to the base station 101 which can receive the downlink radio waves with the highest power. However, since the power of the uplink radio waves from the terminal station 119, received by the base station 101 is low, the request from the terminal station 119 to the base station 101 is not accurately transmitted. It means that the optimum communication cannot be performed. Particularly, in the case of the specification described in the document of the standard of revision TIA/EIA/IS-856, only a base station requested to send data performs the data transmission. Consequently, if the request is not transmitted to the base station 101, the communication cannot be performed at all.
In short, in a situation such that the base station with which the terminal station 119 can perform the optimum communication in the uplink communication and that in the downlink communication are different from each other, a phenomenon that the optimum communication cannot be performed occurs. According to the specification, a phenomenon such that the communication cannot be performed at all occurs.
As one of the solving methods to avoid the situations, a method of transmitting an uplink signal received by the base station 102 via the base station control device 100 which controls a plurality of base stations to the base station 101 can be considered. However, the amount of information transferred between the base station control device 100 and base stations 101 and 102 and the signal process amount of the base station control device 100 increase. A time delay due to the increase in the process amount or redundant transmission may occur, or a case such that the process amount exceeds the process capability and the communication is interrupted may happen.
As another solving method, a method of performing data transmission requested to the base station 101 by the base station 102 can be also considered. In this case, however, since the power received from the base station 102, in the terminal station 119 is low, the transfer rate is lower than that in the case of transmitting data from the base station 102. Further, since the base station for transmitting data is switched, the process amount of the base station control device 100 increases.